Background: The ultrastructural analysis of cultured follicles could direct us to understand subcellular changes during in vitro culture. Objective: This study was done to verify the ultrastructural characteristics of in vitrocultured mouse isolated preantral follicles in co-culture system in the presence andabsence of leukemia inhibitory factor (LIF) Materials and Methods: Mechanically isolated preantral follicles were divided intofour groups: control without LIF, control with LIF, co-cultured group with LIF, coculturedgroup without LIF. In co-culture groups the follicles were cultured withcumulus cells. After 4 days the follicles were processed and sectioned for transmission electron microscopic examination.Results: The oocytes of cultured preantral follicles in all studied groups demonstrated ahomogeneous cytoplasm and they had the round or ovoid shaped mitochondria withlight matrix and cristae. Their endoplasmic reticulum cisternae were in association with mitochondria and Golgi complex. The cortical granules and the aggregation ofmitochondria around the germinal vesicle were prominent in both co-cultured groups. The organelle distribution in granulosa cells was normal in all groups of study and nosign of cell death was observed. In both co-cultured systems the granulosa cellscontained mitochondria with tubular cristae, a well developed smooth endoplasmic reticulum and several large lipid droplets, characteristics of steroid synthesis cells.Conclusion: The oocyte and granulosa cells in co-cultured system showed more remarkable maturation features than that of control.